1) Field of the Invention
In the context of this specification, a stent is a support structure, more or less tubular, for placement within a bodily lumen to support the tissue walls of the lumen. These stents usually require a delivery system to bring them to precisely the desired position within the body of the patient. This invention relates to stent delivery systems.
Two broad categories of stent can be identified. In one category, the stent is fitted around a sausage-shaped balloon, the balloon itself being on the distal end of a catheter. The catheter is advanced, for example, in the arterial system of the patient to the location where the stent is to be placed, and the balloon is then inflated to deform the stent plastically, expanding the stent radially against the wall of the bodily lumen. Since the deformation is plastic, the stent remains in its expanded disposition after deflation of the balloon, and the catheter and balloon system can then be withdrawn.
A second category of stent comprises stents which are self-expanding. For these stents, the delivery system employs some sort of sheath to constrain the stent in a radially small configuration. When the stent is in the desired location, the constraint radially outside the stent is withdrawn, allowing the stent to “spring” radially outwardly to press against the tissue wall of the lumen and permit withdrawal of the delivery system.
The present invention relates to a delivery system for a self-expanding stent. In this specification, the expression “proximal” relates to a point at the end of the delivery system held by the physician, and “distal” to the opposite end.
2. Description of the Related Art
U.S. Pat. No. 5,645,559 (Hachtman et al.) discloses a delivery system for a radially self-expanding stent, the system having an inner tube around which the stent is fitted, and an outer tube that radially compresses the stent. FIGS. 5 to 8 of the drawings of US '559 shows progressive proximal withdrawal of the outer tube so as to release the self-expanding stent progressively along its length commencing with the distal end, and with the extreme proximal end of the stent being the last part of the stent to be released radially outwardly.
US '559 mentions the problem that during this release process there have been instances of axial travel of the stent relative to the delivery system, and not under the control of the surgeon or radiologist, so that the stent can end up in a position in the bodily lumen either proximal of or distal of, the desired location in the lumen. US '559 addresses this problem and proposes as a solution the addition of a relatively soft sleeve element which sits between the stent and the inner tube. This soft sleeve is required to exhibit on its radially outward surface a plurality of circumferential ribs. US '559 stresses that the ribs should be adjacent to the medial portion of the stent. Its FIG. 22 shows a bed 21 for the stent and what appear to be 14 ribs all in the central part of the length of the bed.